A light detector of a conventional optical head includes a light-receiving unit, and a computation circuit for performing a prescribed computation on an electric signal obtained by photoelectrically transducing a light beam received at the light-receiving unit. The light-receiving unit is situated in the vicinity of generally the center of the light detector. On the light beam incident side of the light detector, an aperture is disposed (e.g., see Patent Literature 1).
FIG. 29 is a view showing a configuration of an optical system of a conventional optical head described in Patent Literature 1. FIG. 30 is a view showing the details of a detection optical system of the conventional optical head. FIG. 31 is a view showing a light-receiving surface of the light detector of the conventional optical head.
In FIG. 29, the optical head includes a semiconductor laser 401, a collimator lens 402, a beam splitter 403, an objective lens 404, detection means 406, a light detector 407, an aperture 408, and a diffraction grating 409.
An optical beam emitted from the semiconductor laser 101 is split into a plurality of different light beams by the diffraction grating 409. The light beams which have passed through the diffraction grating 409 are converted into parallel light beams at the collimator lens 402, and pass through the beam splitter 403. The light beams which have passed through the beam splitter 403 are converged by the objective lens 404, resulting in a so-called 3-beam converged light. The converged light is applied to the recording layer of the optical disc 405. A light reflected and diffracted at the recording layer of the optical disc 405 passes through the objective lens 404 again, and is reflected at the beam splitter 403. The objective lens 404 is driven in the optical axis direction (focusing direction) and in the radius direction (radial direction) of the optical disc 405 by an objective lens actuator not shown. The light beam reflected from the beam splitter 403 passes through the detection means 406, and is made incident on the light detector 407. At this step, the aperture 408 formed between the detection means 406 and the light detector 407 intercepts a stray light incident on the light-receiving unit of the light detector 407.
Further, in FIG. 30, the light detector 407 receives a light beam which has passed through the aperture 408. In the aperture 408, one opening 408a is formed. The shape of the opening 408a is generally a circular shape about the optical axis as indicated with a broken line in FIG. 31.
FIG. 32 is a view showing the arrangement of the light-receiving unit on the light-receiving surface of the light detector of the conventional optical head. The light beam which has passed through the detection means 406 is received by a quadrant light-receiving unit 410, so that a so-called focus error signal is generated.
FIG. 33 is a view showing the optical detection system of the conventional optical head. FIG. 34 is a view showing a light beam formed at the quadrant light-receiving unit of the light detector of the conventional optical head. As shown in FIG. 33, the detection means 406 has a cylindrical surface 406a on the light beam incident surface side thereof, and a concave lens surface 406b on the emitting surface side thereof. The detection means 406 causes an astigmatic difference of the difference in focal position by an angle of 90 degrees in a plane orthogonal to the optical axis. Further, the direction of the cylindrical surface 406a is disposed tilted at an angle of generally 45 degrees with respect to the quadrant light-receiving unit 410 of the light detector 407.
The face deflection or the like of the optical disc 405 changes the relative distance between the recording layer of the optical disc 405 and the objective lens 404. As a result, as shown in FIG. 34, the light beam 412a at the focal position becomes in a circular shape, and a light beam 412b at the anterior focal line and a light beam 412c at the posterior focal line are in ellipse shapes orthogonal to each other.
In FIG. 32, by computing the difference between the sum signals of the diagonal light-receiving regions of the quadrant light-receiving unit 410, a so-called focus error signal is detected; and by computing all the light-receiving region sum signals of the quadrant light-receiving unit 410, an RF signal is detected.
Further, the sub-beam light-receiving unit 411 of the light detector 407 receives a sub-beam in a so-called 3-beam method converged on the track of the recording layer of the optical disc 405, and reflected from the recording layer. With a so-called 3-beam method using a so-called push-pull signal computed based on the light quantity of the main beam 412 received at the quadrant light-receiving unit 410, and signals computed based on the light quantities of the sub-beams 413 received at the sub-beam light-receiving units 411, a tracking error signal is generated. Accordingly, there is performed tracking servo causing the objective lens 404 to follow the track of the recording layer of the optical disc 405.
The light detector 407 is previously fixed on a holder (not shown), and further, the optical axis adjustment of the light detector 407 is performed so that the light beam is incident on generally the center of the quadrant light-receiving unit 410. Then, the position of the light detector 407 is determined, and further, the holder and the light detector 407 are fixed on an optical base (not shown). The aperture 408 desirably has the smallest possible dimensions so as to prevent an unnecessary surface reflected light reflected from the surface of the optical base from being incident on the quadrant light-receiving unit 410 or the sub-beam light-receiving units 411. Herein, the hole diameter of the aperture 408 results in a value in view of the relative misalignment between the passing light beam diameter and the light detector 407, the dimensional tolerance of the aperture 408, and the like.
Currently, there has been an expectation for the development of an optical head adaptable to a high recording density multilayer optical disc which is compact in size, and has two or more recording layers. In order to implement the optical head adaptable to the compact and multilayer optical disc, the following configuration is necessary: stray lights reflected from other layers of the optical disc are prevented from being incident on the sub-beam light-receiving units 411 by increasing the so-called lateral magnification of a detection optical system which is the ratio between the focal length of the objective lens and the focal length of the collimator lens of the optical head. In addition, the detection optical system of the going path is required to be downsized. Incidence of stray lights reflected from other layers on the sub-beam light-receiving unit 411 causes an offset in a tracking error signal. Further, interference between lights reflected from the objective own layer and lights reflected from other layers results in fluctuations in DC level of the tracking error signal. This largely deteriorates the performance of tracking servo, so that the recording performance and the reproduction performance are reduced.
In order to implement the configuration of the optical head in which stray lights from other layers of the optical disc are not incident on the sub-beam light-receiving units 411, it becomes essential to increase the lateral magnification of the detection optical system of the optical head, and to increase the distance between the main beam and the sub-beam. However, at this step, the size of the aperture 408 is also increased, so that stray lights become more likely to be incident on the light detector 407. Accordingly, offsets are caused in the focus error signal and the tracking error signal, resulting in large deterioration of the quality of a servo signal and the quality of a reproduction signal. As a result, the recording performance and the reproduction performance are deteriorated. Particularly, the sub-beam has a light quantity which is about 1/10 of that of the main beam. Accordingly, a slight change in light quantity due to interference results in a large fluctuation of the tracking error signal.
Further, an increase in diameter of the opening 408a of the aperture 408 also results in a large reduction of the strength of the holder holding the aperture 408. In order to keep the strength of the holder, it is necessary to increase the dimensions of the holder. Accordingly, the dimensions of the optical head are also increased. As a result, it becomes impossible to implement both of the downsizing of the optical head and the improvement of the reproduction performance. In order to implement both of the downsizing of the optical head and the improvement of the reproduction performance, the following configuration is necessary: stray lights reflected from other layers of the optical disc are prevented from being incident on the sub-beam light-receiving units 411 by increasing the lateral magnification of the detection optical system. In addition, the dimension in the height direction of the optical head is required to be reduced by downsizing the detection optical system of the returning path of the optical disc, and downsizing the optical element and the light-receiving element.
In order to downsize the detection optical system of the returning path, it is necessary to reduce the focal lengths of the objective lens, the collimator lens, and the cylindrical lens, and to downsize various components of the optical head. With the downsizing of the light detector, and the downsizing of the holder holding the aperture 408, the downsizing due to the improvement of the precision of the aperture diameter also becomes essential.
FIG. 35 is a view for illustrating the relationship between the magnification of the detection optical system and the distance between the main beam and the sub-beam on the light detector, and the relationship between the magnification of the detection optical system and the distance between two sub-beams on the light detector. Table 1 is a table for showing the relationship between the magnification of the detection optical system and the distance between the main beam and the sub-beam on the light detector, and the relationship between the magnification of the detection optical system and the distance between two sub-beams on the light detector.
TABLE 1Magnification ofdetection opticalsystem (lateralmagnification β)61416Distance X between120280320main beam andsub-beam (μm)Distance Y between240560640sub-beams (μm)
The lateral magnification of the detection optical system commonly used for a conventional optical head is generally 6 times. When the distance between the main beam and the sub-beam on the optical disc is assumed to be 20 μm, the distance X between the main beam 412 and sub-beam 413 on the light detector 407 becomes 120 μm. On the other hand, when the lateral magnification of the detection optical system is set at 14 times to 16 times in order to reproduce the multilayer optical disc, the distance X between the main beam 412 and the sub-beam 413 on the light detector 407 is increased to 280 μm to 320 μm, resulting in an increase in dimensions of the light detector 407. Further, in order to make a light beam incident so that the sub-beam 413 is not vignetted at the sub-beam light-receiving unit 411 of the light detector 407, it is necessary to increase the diameter of the opening of the aperture. In order to increase the diameter of the opening of the aperture, it is necessary to ensure the strength of the holder for holding the aperture. This has posed a problem that the dimensions of the holder are increased, resulting in an increase in thickness of the optical head in the Y direction of FIG. 30.
Further, in order to minimize the dimension in the Y direction of the light detector 407, when the quadrant light-receiving unit 410 and the sub-beam light-receiving unit 411 are largely spaced apart from each other, a computation circuit is disposed between the quadrant light-receiving unit 410 and the sub-beam light-receiving unit 411. In this case, it results that other-layer stray lights or stray lights reflected from the surface of the optical base and the like are also incident on the computation circuit. When a light is applied to an amplifier unit of the computation circuit of the light detector 407, a weak signal is generated at the amplifier unit. The generated weak signal leaks into an output signal, resulting in an offset of a focus error signal or a tracking error signal. As a result, the qualities of the servo signal and the reproduction signal are largely deteriorated. This results that the recording performance and the reproduction performance are deteriorated.
The focus error signal is calculated based on the following equation (1), and the tracking error signal is calculated based on the following equation (2):Focus error signal=(A2+A4)−(A1+A3)  (1)Tracking error signal=(A3+A4)−(A1+A2)−k(B2−B1)  (2)
Incidentally, in the equations (1) and (2), A1 to A4 each represent the output from each light-receiving region of the quadrant light-receiving unit 410. B1 and B2 each represent the output from each light-receiving region of the sub-beam light-receiving unit 411 divided into two parts, and k represents the gain.
In the tracking error signal, the light quantity of the sub-beam 413 is smaller than the light quantity of the main beam 412. The light quantity of the sub-beam 413 is about 1/10 of the light quantity of the main beam 412. For this reason, the difference value between outputs from respective light-receiving regions of the sub-beam light-receiving unit 411 is multiplied by a gain k, thereby to perform a correction. Generally, the gain k is set at a value of about 1 to 5. At this step, the signal resulting from the sub-beam fluctuates due to interference, resulting in a large fluctuation of the tracking error signal. For this reason, it becomes essential to reduce the quantity of other-layer stray lights incident on the sub-beam light-receiving unit 411.